Envoy, a company that provides an iPad-based sign-in system for office reception desks, has raised $15 million from a single funder: Andreessen Horowitz.

Office reception desks might seem like a dry topic for a startup, but Envoy has seen enviable uptake among hot Silicon Valley companies — and well beyond. Customers include Airbnb (its first customer), Pandora, Heroku, Box, Eventbrite, New Relic, Medium, and Tesla. Andreessen Horowitz was a customer, too, even before it had invested.

Envoy is in use in over 1,000 offices worldwide, and has been used to register over 1 million visitors to date. The customer base has been growing 20 percent every month almost since its founding in February, 2013, says founder and chief executive Larry Gadea.

And the company has done all that, Gadea says, without investing anything in marketing or sales.

“It’s like this patient zero viral kind of thing,” Gadea told me. People use Envoy to sign in when they’re visiting an office, and then they go back to their own office and bring Envoy to their front desks, too.

Andreessen partner Chris Dixon will be joining Envoy’s board.

“Normally a company that got to this stage of growth would have taken some VC money already,” Dixon said. In fact, while this is the company’s first major VC round (its series A), Dixon said the company’s profile — with millions of dollars in revenue — is more typical of a series B.

Gadea was formerly an infrastructure engineer at Twitter and, before that, spent four years at Google doing systems engineering and infrastructure work.

After leaving Twitter to take some time off, he visited a lot of friends at their offices. He noticed that Google, Twitter, and Facebook had their own electronic sign-in systems, designed by their in-house teams, but smaller companies — which lacked the resources to design their own digital receptionists — didn’t.

“Why is it that Google has this solved and nobody else?” he wondered, and the question led him to found Envoy in order to bring a simple office sign-in process to all companies.

Envoy had previously raised $1.15 million from a handful of angel investors, Gadea said, including Salesforce.com founder Marc Benioff and Yelp cofounder Jeremy Stoppelman.

It’s not just for office front desks, either: Dixon thinks Envoy could sell the same or similar technology to gyms, conferences, schools, and even doctor’s offices.

“I think everywhere you go, there should be an iPad there,” Dixon said.

For his part, Gadea is looking to optimize even more parts of the office experience, such as receiving packages, reserving conference rooms, and more.

“We’re looking to expand this into many more products, centered around making the visitors happy, the employees happy, and the receptionists happy,” Gadea say.

“We will take what is normally a really terrible and manual process in the workplace and make it super enjoyable and delightful to do.”

Coming up next: an iPhone app that can notify you if someone is looking for you at your company’s Envoy-enabled front desk. Gadea said this app would also be able to speed up the process of signing in to other companies’ Envoy reception desks, by delivering your particulars straight to Envoy.